1. Field of the Invention
The present invention relates to a raindrop detecting device and a method of selecting wiping mode for a vehicle.
2. Description of Related Art
A wiper wipes raindrop on a windshield, e.g., front windshield, of a vehicle, and a wiper automatic control apparatus automatically controls the wiper. The wiper automatic control apparatus includes a raindrop detecting device having a light-emitting element and a light-receiving element. The light-emitting element emits light, and the windshield reflects the emitted light. The reflected light is introduced into the light-receiving element, and the light-receiving element detects an amount of light received by the light-receiving element. Thus, raindrop amount can be detected, because the amount of the received light is changed in accordance with the raindrop amount. Further, a wiping mode of the wiper can be selected based on the detected raindrop amount.
EP 1 623 896 A1 (corresponding to JP-A-2004-338526) discloses a wiper automatic control apparatus having a raindrop detecting device. The wiper automatic control apparatus is required to smoothly correspond to a rapid change of a rainfall condition caused when a vehicle passes through a tunnel, for example.
The wiper automatic control apparatus determines that the vehicle enters a rainfall-blocked area, i.e., the tunnel. In response to the determination, the wiper automatic control apparatus sets the wiping mode low, relative to a rainfall having a level lower than a predetermined level. In contrast, the wiper automatic control apparatus sets the wiping mode high, relative to a rainfall having a level equal to or higher than the predetermined level.
While a subject vehicle travels through the tunnel, a front vehicle traveling in front of the subject vehicle rolls up minute water drops toward a front windshield of the subject vehicle. The rolled-up minute water drops adhere onto the front windshield of the subject vehicle, and affect a vision of a driver driving the subject vehicle. Therefore, the adhering water drops have to be detected and wiped by a wiper of the subject vehicle.
However, the rolled-up water drop has a relatively small diameter, and an amount of the rolled-up water drops is relatively small. Therefore, the wiping mode is not required to be high for a certain time before the vision of the driver is affected. If the wiping mode is set high relative to the rolled-up water drops, movement of the wiper may be troublesome for the driver before the vision of the driver is affected. That is, in the tunnel, the wiper has to wipe the adhering water drops, but the wiping mode of the wiper has to be set lower.
In contrast, when the subject vehicle travels out of the tunnel, the subject vehicle is exposed to an approximately the same amount of rainfall before the subject vehicle enters the tunnel. Therefore, the wiping mode has to be set similar to that before the subject vehicle enters the tunnel. However, the driver may feel the rainfall to become heavier when the subject vehicle travels out of the tunnel, because the vision of the driver is better in the tunnel due to its rainfall-blocked condition. Therefore, the wiping mode may have to be set higher when the subject vehicle travels out of the tunnel.
However, if the rainfall amount is small when the subject vehicle travels out of the tunnel, the wiping mode may not have to be set higher, because it takes a relatively long time before the vision of the driver is affected. That is, the wiper automatic control apparatus is required to smoothly correspond to the rapid change of the rainfall condition caused when the vehicle passes through the rainfall-blocked area.
Here, when a size of the raindrop detecting device is relatively small, it may be difficult for the small-sized raindrop detecting device to detect raindrop on the whole windshield. Further, in a case where a sensitivity controller provides a sensitivity data of a user of the vehicle to the raindrop detecting device, when the sensitivity controller is set to have a high sensitivity, the wiping mode is easily set high even if the rainfall amount is relatively small, for example. For a variety of users having different sensitivities, the sensitivity controller adjusts a relationship between the detected raindrop amount and the wiping mode.
When the sensitivity controller is set to have the high sensitivity, the wiper is moved at a higher speed, so that a time allowed to detect the raindrop amount becomes shorter. Thereby, it may be more difficult for the raindrop detecting device to accurately detect the raindrop amount. In this case, the wiping mode may be unnecessarily lowered, and movement of the wiper may become unstable.
In order to prevent this disadvantage, the wiping mode is set to be lowered when any raindrop is not detected while the wiper performs a predetermined number of wiping operations. That is, the wiping mode is lowered when the detected raindrop amount is kept to be smaller than a predetermined threshold. Alternatively, an average of the detected raindrop amounts may be calculated after the wiper performs the predetermined number of wiping operations, and the wiping mode may be lowered by comparing the calculated average with a predetermined threshold.
In these cases, although the wiping operation can be stable, responsivity for lowering the wiping mode may be delayed. Further, the responsivity depends on a sense of the user, e.g., driver, of the vehicle so that the responsivity is difficult to be determined to be fast or slow.
In contrast, when the vehicle enters the rainfall-blocked area, the responsivity for lowering the wiping mode is required to be fast, because the rainfall is blocked in the tunnel, for example.